1. Field of the Invention
The present invention relates to a mobile phone with a communication channel switching determining unit suitable for the technique in which unnecessary channel switching can be restrained.
2. Description of the Related Art
A system for a working area such as an office and a factory will be described as an example of a digital cordless phone system. The digital cordless phone system in the working area is generally operated in the system configuration shown in FIG. 1.
The digital cordless phone system shown in FIG. 1 is composed of a switching station 10, base stations 21 to 23 and a mobile station 40. The switching station 10 controls the base stations and the lines. Each of the base stations 21 to 23 performs a radio communication with the mobile station 40. Radio zones 31 to 33 are formed for the respective base stations 21 to 23.
In the digital cordless phone system which has such a structure, the mobile station 40 operates as follows.
Now, it is supposed that the mobile station 40 exists in the radio zone 31 of the base station 21 and it is during communication with the base station 21. As shown by an arrow in FIG. 1, when the mobile station 40 moves away from the base station 21, a reception electric field intensity in the mobile station 40 becomes low. At the same time, the frame error rate becomes high so that the communication quality degrades.
The mobile station 40 performs a channel switching determining process, when the mobile station 40 detects the degradation of the communication quality. At this time, the mobile station 40 selects as a new communication channel, either of another communication channel in the base station 21 and another communication channel in the base station 22 or 23. The communication is continued by use of the selected new communication channel.
Generally, the switching to the other communication channel of the base station for which the communication channel has been established is called a channel switching in the base station (intra-station channel switching). Also, the switching to a communication channel of the other base station is called hand-over (inter-station channel switching).
When the mobile station 40 selects the channel switching to a new communication channel in the base station as the result of the channel switching determining process, the mobile station 40 measures an interference wave to the new communication channel in the base station 21, i.e., performs carrier sense. The new communication channel with the base station 21 is established, when it is determined that the channel is usable. The communication with the base station 21 by use of a new communication channel is performed through the establishment of the new communication channel and the communication is continued.
On the other hand, when the hand-over is selected for the channel switching, the mobile station 40 first searches one of the base stations 21 to 23 having the highest one of the reception signal levels higher than a threshold value. In FIG. 1, it is supposed that the reception signal level from the base station 22 is the highest, because the mobile station 40 is positioned on a position the closest to the base station 22. The mobile station 40 performs the establishment of synchronization with the base station 22, when the base station 22 with the highest reception electric field intensity level is searched. When the synchronization establishment succeeds, the carrier sense of the communication channel specified by the base station 22 is performed and the communication channel is established. The channel switching from the base station 21 to the base station 22 is performed as the result of the establishment of the communication channel and the communication is continued.
In a case where the mobile station moves during communication, the communication quality is degraded with the moving away of the mobile station from the base station in the communication state as described above. Also, even if the mobile station stops at a position and continues the communication, the communication quality is degraded when any interference wave against the communication channel is generated. Therefore, in such cases, the communication channel switching would become necessary.
The mobile station always monitors the frame error rate of the communication channel during the communication, and detects the degradation of the communication quality based on this frame error rate. Also, in the channel switching determining process when the degradation of the communication quality is detected, the frame error rate and the reception electric field intensity levels of the communication channel become the determination condition for the selection of a channel switching destination. That is, the communication channel switching in the mobile station is performed through the selection of the channel switching destination on the basis of the frame error rate, the communication quality degradation detection derived from the frame error rate, and the reception electric field intensity level.
However, in the above conventional technology, stable communication continuation becomes difficult, in a case where the system is installed in the factory in which an equipment is located around the system to generate the electromagnetic waves to function as any cause of the communication channel interference. In this case, the communication quality degradation is detected and the mobile station repeats the channel switching frequently under the environment in which the interference wave is generated from the equipment in an impulse manner to interfere with the radio communication channel.
As described above, in the channel switching, a communication channel is established after the carrier sense of the switched channel is performed. However, in case that an interference wave is not a continuous manner but an impulse manner, the channel establishment sometimes fails due to the interference wave generated when the communication channel is established, even if it is determined at the time of the carrier sense that the interference wave is not present. Especially, when a hand-over process occurs frequently for the channel switching, the establishment of the synchronization sometimes fails due to the interference wave generated in an impulse manner. As a result, it becomes more difficult to provide the stable communication due to the frequent occurrence of interference such as the communication breaking-off. Therefore, the channel switching operation should be restrained to the degree that the minimum communication quality can be secured, under the environment in which the synchronization establishment and the communication channel establishment are difficult.
In this case, the communication breaking-off can be prevented so that the communication is possible to be stabilized.
In conjunction with the above description, a radio channel switching system in a mobile station is described in Japanese Laid Open Patent Application (JP-A-Heisei 1-120135). In this reference, when the degradation of communication quality is detected while a mobile station communicates with a radio base station using a first radio channel, the mobile station selects another base station to establish a second radio channel. Then, the mobile station transmits a data signal through said first and second radio channels while said first and second radio channels are alternately switched for a switching transmission and reception time interval. The mobile station switches a use radio channel from the first radio channel to the second radio channel, after the communication quality is confirmed in the second radio channel. In this case, the same channel is used in a time divisional manner to prevent the interference to another mobile station which using the same channel.
Also, a mobile communication station is described in Japanese Laid Open Patent Application (JP-A-Heisei 1-183221). In this reference, a check of a new communication channel is performed as follows. While a communication is performed using a time divisional multiple communication channel, a frequency is switched to a new frequency for the new communication channel after a time slot of the communication channel is ended. The channel is again switched to the communication channel when the check of the new communication channel is ended or when a predetermined time interval elapses.
Also, a mobile communication channel switching control system is described in Japanese Laid Open Patent Application (JP-A-Heisei 4-144429). In this reference, a plurality of base stations form radio zones. When a mobile station moves between the plurality of radio zones, the mobile station can continue the communication by referring to a peripheral base station zone table which indicates destination base stations and by switching a channel to a new channel for a selected one of the destination base stations. The selected destination base station satisfies a predetermined channel switching condition. At this time, a priority level is allocated to each of the destination base stations, and one of the destination base stations is selected to satisfy the predetermined channel switching condition in accordance with the priority levels.
Also, a communication switching system in a mobile communication system is described in Japanese Laid Open Patent Application (JP-A-Heisei 8-256369). In this reference, a mobile station (21) performs connection of a communication channel to a destination base station (12) using a time slot which is not used for the communication with a source base station (11). Then, the mobile station switches the communication channel from the channel to the source base station (12) to the channel to the destination base station (12), and then release the channel to the source base station (12). At the same time, a communication path to a counter station (31) is connected to the source and destination base stations (11 and 12), and then the communication path connected to the source base station (11) is released.
Also, a control channel switching system is described in Japanese Laid Open Patent Application (JP-A-Heisei 8-289353). In this reference, a TDMA-MCA system radio communication system has a (mxc3x97n) channel structure which has m carriers different in frequency and one carrier is divided into n time slots. When a failure occurs in a current communication channel as the control channel (S1), the control channel is shifted to a communication channel which is not used for communication (S3). If it is impossible, after the disconnection of the communication channel and the automatic re-connection are informed previously (S5), the communication channel is disconnected and the control channel is shifted to the communication channel (S6). A call for the disconnected communication channel is allocated to the head of a connection queue for the re-connection (S7). Thereafter, the call of the disconnected communication channel is connected to the channel in which the communication is ended (S8 and S9).
An object of the present invention is to provide a mobile phone in which communication can be stabilized.
Another object of the present invention is to provide a mobile phone in which channel switching can be restrained such that communication is stabilized.
Still another object of the present invention is to provide a mobile phone in which communication is stabilized, even in the environment in which an interference wave to a communication channel is frequently generated.
In order to achieve an aspect of the present invention, a mobile phone includes an error detecting section and a channel switching control section. The error detecting section detects a frame error rate on a present communication of one of a plurality of base stations. The channel switching control section determines whether a communication quality is degraded, based on the detected frame error rate and a past channel switching result. Also, the channel switching control section determines that a channel switching from the present communication channel to a new communication channel should be performed, when it is determined that the communication quality is degraded.
The channel switching control section may determine a frame error rate threshold value based on the past channel switching result, and compare the detected frame error rate and the determined frame error rate threshold value. Then, the channel switching control section may determine that the communication quality is degraded when the detected frame error rate is equal to or larger than the determined frame error rate threshold value. In this case, the channel switching control section may set a first threshold value as the determined frame error rate threshold value, when the past channel switching result indicates that a success rate of past channel switching operations is equal to or higher than a success rate threshold value. Also, the channel switching control section may set a second threshold value as the determined frame error rate threshold value when the past channel switching result indicates that the success rate of the past channel switching operations is lower than the success rate threshold value. Here, the second threshold value is larger than the first threshold value. The past channel switching operations may be hand-over operations, or intra-station channel switching operations.
The channel switching control section may calculate the success rate of the past channel switching operations, when the past channel switching operation is executed a predetermined number of times.
The channel switching control section may set one of first and second threshold values as the determined frame error rate threshold value based on the past channel switching result. Here, the second threshold value is larger than the first threshold value. In this case, the past channel switching result may include a first success rate of hand-over operations and a second success rate of intra-station channel switching operations. The second threshold value is preferably set when the first success rate is lower than a first success rate threshold value, or when the first success rate is equal to or higher than the first success rate threshold value but the second success rate is lower than a second success rate threshold value. Also, the first threshold value is preferably set when the second success rate is equal to or higher than the second success rate threshold value. The channel switching control section may calculate the first success rate when a hand-over operation is executed a first predetermined number of times, and the second success rate when an intra-station channel switching operation is executed a second predetermined number of times.
The mobile phone may further include an intensity detecting section detecting reception electric field intensities from signals received from the plurality of base stations. At this time, the channel switching control section may determine whether a hand-over process or an intra-station channel switching operation should be performed, based on the detected reception electric field intensities, when it is determined that the channel switching should be performed.
Also, the mobile phone may further includes an intensity detecting section detecting reception electric field intensities from signals received from the plurality of base stations. In this case, the channel switching control section includes a channel switching determining section, a setting section and a state monitoring section. The channel switching determining section determines whether the communication quality is degraded, based on the detected frame error rate and a frame error rate threshold value. Also, the channel switching determining section determines that the channel switching from the present communication channel to the new communication channel should be performed based on a channel switching method, when it is determined that the communication quality is degraded. The setting section selecting one of a first threshold value and a second threshold value as the frame error rate threshold value in response to the past channel switching result to set to the channel switching determining section. Here, the first threshold value is smaller than the second threshold value. The state monitoring section monitors results of past channel switching operations to output the past channel switching result to the setting section. In this case, the channel switching determining section determines that the communication quality is degraded, when the detected frame error rate is equal to or larger than the frame error rate threshold value. Also, the channel switching determining section determines which of a hand-over process or an intra-station channel switching process should be performed, based on the detected reception electric field intensities, when it is determined that the communication quality is degraded.
The state monitoring section monitors a success rate of the hand-over processes, and determines whether a hand-over failure does not occur frequently, based on the hand-over success rate and a success rate threshold value. Also, the state monitoring section outputs the past channel switching result to the setting section based on the determining result of whether a hand-over failure does not occur frequently. In this case, the setting section selects the second threshold value as the frame error rate threshold value, when it is determined by the state monitoring section that the hand-over failure occurs frequently. Also, the setting section sets the first threshold value as the frame error rate threshold value, when it is determined by the state monitoring section that the hand-over failure does not occurs frequently. The second threshold value is set to a value in a range with an upper limit value such that a communication can be performed on the present communication channel at least.
Also, the state monitoring section sets the success rate of the hand-over processes is increased, when synchronization with a destination one of the plurality of base stations is established at once hand-over process.
The state monitoring section includes a hand-over monitoring section monitoring an execution result of the hand-over process, and an intra-station channel switching monitoring section monitoring an execution result of the intra-station channel switching process. The state monitoring section outputs at least one of the execution result of the hand-over process and execution result of the intra-station channel switching as the past channel switching result.
In this case, the hand-over monitoring section may monitor a number of times of execution of the hand-over process and the execution result of the hand-over process. Also, the hand-over monitoring section may calculate the success rate of the hand-over processes after a predetermined number of times of the execution of the hand-over process, and determine whether or not the hand-over process occurs frequently. Also, the intra-station channel switching monitoring section may monitor a number of times of execution of the intra-station channel switching process and a communication channel establishment result in the intra-station channel switching process. Then, the intra-station channel switching monitoring section may calculate a communication channel establishment success rate after a predetermined number of times of the execution of the intra-station channel switching process, and determine whether or not the communication channel is successfully established.